1. Field of the Invention
The present invention relates to a method of handling mobility in multimedia broadcast multicast service single frequency network (MBSFN) in a wireless communication system and related communication device, and more particularly, to a method of handling mobility in MBSFN for improving MBMS service continuity and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs). The LTE protocol stack can be segmented into access stratum (AS) layer and non-access stratum (NAS) layer. The AS layer includes sublayers as Layer 3, also known as the Radio Resource Control (RRC) layer, Layer 2, consisting of three sub-layers that are the Packet Data Convergence Protocol (PDCP) layer, the Radio Link Control (RLC) layer, and the Medium Access Control (MAC) layer, and Layer 1, also known as the Physical (PHY) layer. The NAS layer processes the signaling between the UE and the core network.
Recently, the 3GPP is involved in the further advancements for E-UTRA and proposes an LTE-Advanced system as an enhancement of the LTE system. Carrier aggregation, where two or more component carriers are aggregated, is introduced into the LTE-Advanced system in order to support wider transmission bandwidths, e.g. up to 100 MHz and for spectrum aggregation. A UE of the LTE-Advanced system can simultaneously receive and/or transmit on multiple component carriers. Carrier aggregation allows a UE to aggregate a different number of component carriers of possibly different bandwidths in uplink and downlink.
Evolved multimedia broadcast multicast service (E-MBMS) has been introduced in the LTE specification to broadcast or multicast TV, films, information such as free overnight transmission of newspaper in a digital form. Two important scenarios have identified for the E-MBMS: one is single-cell broadcast, and the other is MBMS single frequency network (MBSFN). The MBSFN is a simulcast transmission technique that realizes transmission of identical waveforms at the same time from multiple cells. Since the MBSFN transmissions from the multiple cells are closely time-synchronized, the MBSFN transmission arrived at the UE is regarded as a transmission from a single cell and the UE may treat the MBSFN transmission in the same way as multi-path components of a single cell transmission without additional complexity. An eNB belongs to only one MBSFN area and there is no overlapping between different MBSFN areas.
To realize E-MBMS, a downlink transport channel is required to be broadcasted in the entire coverage area of each cell in the MBSFN area, which is called a multicast channel (MCH). The MAC layer of the LTE system offers data transfer services between the MCH and logical channels including a multicast traffic channel (MTCH) and a multicast control channel (MCCH), which are point-to-multipoint channels for transmitting traffic data and control information. An MCCH is utilized for each MBSFN area and carries a message which lists all the MBMS services with ongoing sessions, transmitted by all cells within a corresponding MBSFN area except the reserved cell which does not contribute to the MBSFN transmission. MCCH change notification is done by an MBMS radio network temporary identifier (M-RNTI) on the physical downlink control channel (PDCCH).
For network controlled mobility in the RRC_CONNECTED mode, the E-UTRAN triggers a handover procedure based on radio conditions and load, and thereby decides which cell a UE should hand over to in order to maintain the radio link. The source eNB (which controls the source cell) requests the target eNB to prepare for the handover and the target eNB subsequently generates a handover message to order the UE to perform the handover. After receiving the handover message, the UE attempts to access the target cell according to a random access resource selection procedure, and upon successful completion of the handover, the UE sends a message to confirm the handover.
UEs that are receiving MBMS services in the RRC_IDLE mode performing cell reselection or are in the RRC_CONNECTED mode can obtain target cell MTCH information from the MCCH of the target cell. However, upon handover, mechanisms to deliver MCCH to UEs via a handover message are not supported. No new information is provided to help a UE in switching MBMS reception between MBSFN areas. Up to now, it is not clear in the specifications that how a UE continues MBMS reception when the UE performs handover from a source cell in an MBSFN area to a target cell in the same MBSFN area or different MBSFN area in which the same MBMS service is also provided. Also, it is not clear that which is responsible for indicating that the UE has already left the MBSFN area.
With the addition of carrier aggregation and the possibility of MBSFN dedicated frequency, it is predictable that an eNB may be associated with multiple MBSFN areas, which results in impact on UE mobility in MBSFN, including the mechanism of MCCH change notification and MBMS service continuity.